The SV40 t antigen is a 20 Kd nuclear + cytoplasmic protein which is active in the process of virus-induced neoplastic transformation. However, there is little information on how it functions in vivo. In the next grant period, we propose to search for evidence that it forms in vivo complexes with three specific cell proteins, each of which can bind specifically to t in vitro. We will also attempt to establish useful, new functional assays for t and to perform detailed genetic and in vitro chemical analyses of its structure/function relationships. Ideally, results of such studies will provide new insights into how t operates biochemically and suggest a mechanism by which it operates in the viral transforming process.